1. Field of the Invention
The present invention relates to a pushbutton switch key sheet for use in operating portions of various electronic apparatuses, such as a mobile phone, a PDA, a car navigation apparatus, and a car audio apparatus, and more particularly, to a key sheet and pushbutton switch in which a plurality of key tops are exposed through an operational opening having no partition frame formed in a casing of an electronic apparatus.
2. Description of the Related Art
For example, for use in a mobile phone 1, there is known, as shown in FIG. 13, a pushbutton switch in which, for a reduction in size of an operating portion and an improvement in terms of an artistic design of the pushbutton switch, a plurality of key tops 4 of a key sheet 3 are exposed in a dense state through an operational opening 2a with no partition frame, which is formed in a casing 2. The number of key tops 4 are seventeen in total, an interval between the adjacent key tops 4 is very small, for example, the interval is set to approximately 0.15 mm to 0.2 mm, and a clearance between the key tops 4 and the operational opening 2a is also set to a very small dimension.
For example, in JP 2004-319396 A, as shown in FIG. 14, the conventional key sheet 3 is formed by fixing the key tops 4 to the operating surface side surface of a base sheet 5. The base sheet 5 is equipped with base portions 6 to which the key tops 4 are fixed and which are formed of a silicone rubber and a reinforcing member 7 formed of a resin. Thus, if the key sheet 3 is set upright or tilted during use of the mobile phone 1, it is possible to suppress overall distortion of the key sheet 3 due to the rigidity of the reinforcing member 7. Each base portion 6 is composed of a pedestal portion 6a to which the key top 4 is fixed, and a thin-walled flexible portion 6b elastically supporting the pedestal portion 6a. 
In the above key sheet 3, gaps corresponding to the height of the pedestal portions 6a are formed in the outer edges on the back side of the key tops 4. Since the casing 2 of the mobile phone 1 has no partition frame, there is a possibility of fingernails or the like entering the gaps during depressing operation on the key tops 4, and there is a fear of the key tops 4 being unstuck or detached from the base sheet 5.